Spot welding is a known welding technology for welding together metal plates, which can be used in the manufacture of automobiles, domestic electrical products, and the like. In spot welding, firstly, as illustrated in FIG. 23, two superimposed metal plates 100a, 100b are sandwiched between a pair of electrodes 150a, 150b. In this state, pressure is applied locally to the metal plates 100a, 100b by means of the pair of electrodes 150a, 150b, and current is passed between the electrodes 150a, 150b. The current flows in a concentrated manner through the portion of the metal plates 100a, 100b sandwiched between the electrodes 150a, 150b, and therefore generates Joule heat. A portion of the metal plates 100a, 100b is melted by this Joule heat, whereupon, the passage of current is halted. When the molten portion of the metal plates 100a, 100b cools and solidifies, the metal plates 100a, 100b will be welded together.
FIG. 24 is a cross-sectional view of a spot welded section of two metal plates 100a, 100b which have been spot welded as described above. In the spot welded section, the outer surfaces of the metal plates 100a, 100b are dented due to the pressure applied by the electrodes 150a, 150b. This denting is called an “indentation” 101, and the length L1 thereof is called the “indentation diameter”. A nugget section 102 and a pressure bonded section 103 are formed in the spot weld section. The nugget section 102 in a region where the metal plates 100a, 100b have become unified as a result of being melted due to the application of heat and pressure, and then solidifying. The length L2 of the nugget section 102 is called the “nugget diameter”. This nugget diameter L2 greatly influences the welding strength achieved in the spot weld section. The greater the nugget diameter L2, the greater the weld strength of the spot weld section. The pressure bonded section 103 is a region which has received the effects of the applied heat and applied pressure and where the metal plates 100a, 100b have bonded together under pressure. Together, the nugget section 102 and the pressure bonded section 103 are known as the “joint section 104”, and the length L3 of this joint section 104 is called the joint diameter. The original material 105 surrounding the joint section 104 is a region which does not contribute to the joint strength of the spot weld.
Generally, the nugget diameter L2 or the pressure bond diameter L3 in the spot weld section achieved by welding is appropriately 10 millimeters or less, which is relatively small. Therefore, in many cases, it is necessary to inspect the spot weld section in order to check that it has sufficient weld strength. Since the weld strength of the spot weld section is greatly influenced by the nugget diameter L2, then the nugget diameter L2 can be used effectively as a basis for judging whether or not the spot weld section has a suitable welded state.
Japanese Patent Laid-open No. Hei10-26609 discloses inspection technology, one object of which is to measure the nugget diameter L2 in a non-destructive manner, and to judge the suitability or unsuitability of the welded state of a spot weld section on the basis of these measurement results. According to this patent publication, an exciting coil is disposed in the vicinity of an inspection target, and a loop coil forming a sensor is disposed between the inspection target and the exciting coil. In this state, a static magnetic field is generated which passes through the inspection target and the sensor, by passing a DC current through the exciting coil. Thereupon, when the static magnetic field is shut off, the inductance of the loop coil (or a physical quantity that is directly proportional to the inductance thereof) is determined by tracing the course of the loss of the electrical field remaining in the inspection target. This inductance indicates the magnetic permeability of the nugget section 102 and pressure bonded section 103, or the like, constituting the spot welding section through which the residual magnetic field passes. When measurement of this kind is carried out in a plurality of positions with respect to the inspection target, then variation will occur in the plurality of inductances obtained. This variation in inductance reflects variations in the internal structure of the spot weld section. Therefore, the nugget diameter L2 can be estimated by detecting the variations in magnetic permeability, and hence the variations in inductance, caused by changes in the internal structure of the spot weld section, by means of non-destructive inspection technology.
FIG. 25 shows a conventional non-destructive inspection device X2 for executing a non-destructive inspection method as described above. The non-destructive inspection device X2 comprises an exciting pole 210, an exciting coil 220 wound about this pole, a recovering pole 230, a connecting section 240 connecting the exciting pole 210 and the recovering pole 230, and a coil array 250 disposed in the vicinity of the exciting pole 210.
The exciting pole 210 is an iron core for raising the magnetic flux intensity of the magnetic field induced when a current flows in the exciting coil 220, and it is formed integrally with the recovering pole 230, by means of the connecting section 240. The exciting pole 210 has a finely pointed end and has a magnetic flux exciting surface 211 formed on the front end thereof. The magnetic flux exciting surface 211 is a face disposed opposing the inspection target, via the coil array 250. The exciting coil 220 is connected to a drive circuit (not illustrated) which incorporates a DC power supply, a switch and a prescribed resistance. The recovering pole 230 has a recovering surface 231 on the front end thereof. The magnetic flux excited from the magnetic flux exciting surface 211 of the exciting pole 210 is recovered by the recovering surface 231.
The coil array 250 serves to detect magnetic changes in the vicinity of the inspection target and output same in the form of a voltage, and consists of a prescribed number of loop coils 251 disposed sequentially in a position opposing the magnetic flux exciting surface 211. The loop coils 251 are made from a conducting material, such as Cu, and are patterned onto a flexible substrate (not illustrated).
FIG. 26 includes a cross-sectional view along line XXVI—XXVI in FIG. 25, showing a state where the switch of the drive circuit has been switched on and the voltage output by the DC power supply is applied to the exciting coil 220, thereby causing a static magnetic field F3 to be applied to the spot weld section of a steel plate 110. The internal portion of the steel plate 110 that the magnetic flux passes through is magnetized in accordance with the static magnetic field F3. The non-destructive inspection device X2 is composed in such a manner that it traces the course of the disappearance of the residual magnetic field in the magnetized location, after the static magnetic field F3 has been shut off, by means of various sensor coils 251, whereby it is possible to measure the time constant in the transient change of each course of disappearance of the magnetic flux.
However, in a conventional non-destructive inspection device X2, the distance of separation L4 between the magnetic flux exciting surface 211 and the magnetic recovering surface 231 is approximately 3 mm, which is relatively long. If the distance of separation L4 between the magnetic flux exciting surface 211 and the magnetic recovering surface 231 L4 is long, then after the static magnetic field F3 has been shut off, the distance traveled by the residual magnetic field passing through the regions other than the nugget section 102 also becomes longer, accordingly. Therefore, noise is liable to be introduced into the course of disappearance of the residual magnetic field, and hence there are cases where adequate information relating to the nugget section 102 cannot be obtained, from the viewpoints of accuracy and the S/N ratio.
Moreover, in the conventional non-destructive inspection device X2, the respective sensor coils 251 constituting the coil array 250 are provided throughout the whole length of the magnetic flux exciting surface 211, in the width direction L5 thereof. In a composition of this kind, even after the static magnetic field F3 has been shut off, a component of the course of disappearance of the residual magnetic field corresponding to the magnetic field F3 that did not pass through the nugget section 102 will still be detected, which is undesirable from the standpoints of accuracy and S/N ratio.
Furthermore, the width L5 of the magnetic flux exciting surface 211 of the conventional non-destructive inspection device X2 is formed to a narrower width than the width L6 of the exciting pole 210, and the surface area from which the magnetic flux is excited is small. Therefore, when the static magnetic field F3 is applied, the magnetic flux does not pass readily through the nugget section 102, in other words, cases where the spot weld section cannot be magnetized satisfactorily will occur.
In this way, the conventional non-destructive inspection device X2 entails problems in the magnetization of the spot weld section when applying the static magnetic field F3, and the detection of the residual magnetic field after the static magnetic field F3 has been shut off. Therefore, for example, there have been cases where it has not been possible to obtain sufficiently reliable results for the measurement of the nugget diameter L2 in a spot weld section, or the like.